An aqueous substance that covers the anterior surface of the eyeball, keeping the cornea wet.
Comment:
[function-note] "maintenance of the avascular transparent corneal epithelium"; [structure-note] "The tear film is structurally complex with three distinct layers: a surface lipid layer, a middle aqueous layer, and an inner mucus layer[http://physiologyonline.physiology.org/content/13/2/97]"